Uranium is the most valuable energy material, 1 kg of uranium can generate ca. 500,000 megajoule of heat value comparison the same amount of coal only produces ca. 25 megajoules of heat value. Currently, developed and developing countries greatly rely on the nuclear power plants for electricity supply. In the United States, the nuclear energy plants supply ca. 20% of electricity. With the increasing fear to globe warming and shortage of foreign fuel, the solutions in energy security and energy independence are to expand the nuclear fission utilization and develop sustainable energy.
The harmful by-products derived from nuclear power plants during the electricity generation are radionuclides, including transuranic elements (TRU) with extremely long-lived isotopes, radioactive cesium and strontium, actinides, rare earth elements and lanthanides. Certainly, the hazardous materials in the radioactive liquid wastes need be reprocessed before depository in order to meet the government's requirement for diminishing the waste volume with the intention to avoid the second construction of repository. There is no current massive reprocessing activity in the United States, and most of spent nuclear wastes are stored in pools or nearby the reactor sites, for example, the Hanford Nuclear Reservation accumulates around 53 million gallons of nuclear waste. However, the nuclear wastes produced in foreign countries have been specifically treated to removal the hazardous radionuclides by using PUREX process or UNEX process. The largest reprocessing factory in the world is La Hague site located in France where annually ca. 1700 tonnes of nuclear spent fuel has been reprocessed; the rest of reprocessing plants are located in the United Kingdom, Japan, Russia and China.
Radioactive liquid wastes, including high level, intermediate level and low level wastes, can be categorized as two kinds of high level waste, namely fission products and transuranics, which must be managed to remove the harmful radioisotope nuclides before disposal or reuse.
Some of these high-level radioactive aqueous solutions have to be geologically disposed in long term storage (for example Yucca Mountain) due to the long lived elements, such as actinides, lanthanides, 90Sr and 137Cs. Management of long-lived radionuclides-containing wastes has offered advantages to develop unique separation technologies to meet the government policy requirement. Apparently, in order to minimize the capacity of waste, the imperative task is to develop extractant or ligand capable of extracting radioactive elements from acidic or high salinity solution in high selectivity and efficiency.
The TRUEX liquid-liquid extraction has been applied for the recovery of lanthanides and actinides from nuclear waste by utilizing the organophosphorous ligand, octyl-phenyl-N,N-diisobutyl-carbamoylmethyl-phosphine oxide (U.S. Pat. No. 4,548,790/1985).
The PUREX process is used to extract plutonium and uranium from the reprocessing spent nuclear fuel and from one another by liquid-liquid extraction. The active organic extractant (30% tributyl phosphate dissolving in kerosene, see U.S. Pat. No. 4,574,072/1986) transfers the plutonium and uranium from aqueous acidic spent nuclear fuel to organic phase.
Also alkyl-functional phosphoric acid derivatives can complex actinides and lanthanides (Ionova et al., New J. Chem., 2001, 25, 491-501).
Ligands contain only CHNO (carbon, hydrogen, nitrogen and oxygen) elements give the opportunities to avoid the additional pollution during the burn-up process. Currently, DIAMEX process (Sep. Sci. Techol., 2007, 42, 439-452) uses N,N′-dimethyl-N,N′-dibutyl-tetradecylmalonamide as extractant for separating actinides and lanthanides from nuclear waste by a solvation process. Besides the advantage of without the secondary waste, malonamide derivatives have confirmed their excellent complex efficiencies.
Crown Ether and polyethylene glycol derivatives have been applied to remove some radioisotopes, particularly Sr, Cs and small amount of actinides which are remaining during the reprocessing of used spent nuclear fuel.
As mentioned above, removal of radioactive cesium and strontium, as well as minor actinides is particularly vital in the reprocessing of nuclear waste raffinates. More than two decades ago, scientists have started to exploit the ionic halogenated derivatives of cobalt dicarbollide possessing excellent capability to extract 137Cs as the extractant to extract the acidic waste raffinates, and subsequently realize the importance when combining with polyethylene glycol (PEG), since the complex extractant system of (COSAN-PEG) or (CCD-PEG) can be used for synergistically extracting strontium.
A recent industrial liquid-liquid extraction process has been developed in Russia by using the combining extractant system (CCD-PEG), called “UNEX”, to remove the radioactive waste of Cs, Sr and minor actinides. The organic solvents used in “UNEX” are nitrobenzene or chloro-hydrocarbons.
U.S. Pat. No. 6,270,737 (Law et al from Idaho Engineering and Environmental Laboratory) disclosed the method to removal hazardous materials, such as radioactive Cs and Sr, by using non-aromatic solvent and the safe stripping agents. The method involved a synergistic extraction system comprising organoboron (for example chlorinated cobalt dicarbollide) and polyethylene glycol, the diluent solvents are the mixture of bis(tetrafluoropropyl ether of diethylene glycol) with bis(tetrafluoropropyl ether of ethylene glycol) et al.
The European Atomic Commission has recently launched a multinational project for the treatment of spent nuclear waste, called “EUROPART”. “EUROPART” has devoted many contributions to the nuclear waste treatment technologies, for example Calix-CMPO (CMPO referred as carbamoylmethylphosphine oxides, Calix referred as calixarenes) and Calix-Crown. Interestedly, several groups from Europe have demonstrated the ideas by introducing cobalt dicarbollide (COSAN) derivatives to the frame of macrocyclic calxarenes (Mikulasek et al. Chem. Commun., 2006, 4001-4003), as well as incorporate the COSAN and CMPO on the same molecules (Dam et al., Chem. Soc. Rev. 2007, 36, 367-377; Gruner et al. New J. Chem., 2002, 26, 1519-1527; Reinoso-Garcia et al. New J. Chem., 2006, 30, 1480-1492) for the treatment of nuclear waste. Unfortunately, all of those developments are still under the usage of organic solvent (such as nitrobenzene and dichloroethane et al) for the liquid-liquid extractions.
A ready to scale-up achievement including the using chlorinated cobalt dicarbollide (CCD) derivatives and polyethylene glycol (PEG) as one of the series of flowsheets has been developed at Argonne National Laboratory, called “UREX+” process. The advantage of this method is run sequentially in the same shielded-cell contactor and gloveboxes with segments of 1) CCD-PEG (removing Cs and Sr), 2) NPEX (removing Pu and Np), 3) TRUEX (removing Am, Cm, and rare earth), and 4) Cyanex-301 (separating Am and Cm from the heavy rare earths). In the stage of CCD-PEG, the organic solvent for solubilizing CCD-PEG is phenyltrifluoromethyl sulfone.
Calix[4]arene based extractants which attach carbamoylmethylphosphine oxide (hereafter, referred as CMPO) groups on their wide or narrow rims leads to more than two orders efficiency increase compared to octyl-phenyl-N,N-diisobutyl-carbamoylmethyl-phosphine oxide, also increase the selectivity to actinides and lanthanides (Delmau et al., Chem. Commun. 1998, 1627-1628).
US Patent Application No. 2001/6312653 B1 discloses a functionalized calixarenes with CMPO in the wide rim for using in the extraction of actinides and lanthanides. A phosphinoxidoacetamide moieties attached to the frame of the macrocyclic calixarene, this discovery is effective to remove actinides and lanthanides from the radioactive aqueous waste by liquid-liquid extraction.
US Patent Application No. 2006/0205920 A1 discloses a method to introduce CMPO groups to the terminal amino of dendrimers, such as PAMAM for extracting lanthanides and actinides. Those dendrimers (2nd, 3rd, 4th and 5th generations) usually have multiple terminal amino groups which are readily functionalized by amide reaction for the purpose of introduction of CMPO substituents.
US Patent Application No. 2005/6843921 B1 discloses a method, which use the silica absorbent containing organophosphorus, such as octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide on the complex-carrier, for separating and recovering the radioactive liquid wastes. By the elution steps, the absorptive elements on the porous silica carrier particles are eluted separately.
US Patent No. 2001/6258333 B1 discloses an extracting method comprising an extracting agent composition for the treatment of nuclear waste which containing radioactive elements and rare earths by two phases liquid-liquid extractions. By utilizing an organoboron complex, organophosphorus and polyethylene glycol compounds, the selective sequential extraction processes were used for the recovery of radionuclides, also including rare earths and actinides.
US Patent No. 1995/5468456 discloses the using of magnetic particles which contacted the metals-containing liquids to concentrate the metals to the surface of the magnetic particles.
Other specifically designed ligands, for instance polyethylene glycol and crown ether functionalized calixarenes, namely calixcrowns, have developed (Gorbunova et al., Tetrahedron Lett. 2003, 44 (29), 5397-5401, WO94/24138) for approaching the separation of cesium, strontium and actinides.
SANEX process has been created for removal lanthanides to avoid poisoning a neutron driven nuclear reaction. French CEA is studying the extraction efficiency of using N-donating extractants, such as triazine-pyridine derivatives; one of the purpose is to extract Americium (Dam et al., Chem. Soc. Rev. 2007, 36, 367-377).
Carbon nanotubes (hereafter, referred as CNT) have high surface area, high electrical conductivity, high thermal conductivity and high mechanical strength. Owing to its exceptional chemical/physical stability, CNT has been used in harsh/corrosive environments, in particularly; CNT possesses extremely high resistance to acidic environment, for example HNO3 aqueous solution, which allows to be used for the recovery of spent nuclear fuel.
The solubility of CNT can be improved by covalent and/or non-covalent modification. Harsh acid oxidative treatment of CNT by H2SO4/HNO3 mixture produces oxidative function, also short residue of CNT (<200 nm length).